Prey size is determined one of several ways:
1. feed your snake a rodent that is the same width as your snake at the widest point.
2. feed your snake a rodent that is 10-15% of its body weight
3. feed multiple prey items to get the 10-15%.
Personally, i feed my snakes an appropriately sized prey item. (1 above). My ball python, is 1000 grams and is fed weaned or small rats. They leave a slight lump in her after she has fed.
A snake may try to eat a prey item that is too large for it, but it may not. It really depends on the individual animal.